1. Field of the Invention
The invention relates to a scanning antenna diversity system for FM radio for vehicles, having an antenna system with a controllable logic switching device, in which a different high-frequency reception signal, in terms of diversity, is passed to a receiver with different switching positions, in each instance, and an intermediate frequency signal derived from this reception signal turns on a diversity processor, which switches the logic switching device into different switching position if there is reception interference.
2. The Prior Art
Switching arrangements for antenna systems are generally known from German patents DE 35 17 247 A1, and DE 101 02 616 A1. In the case of the antenna diversity reception systems for the elimination of interference during the reception of frequency-modulated radio broadcasts described there, a number of antenna signals are passed to a diversity processor, wherein a selected antenna signal is switched through to the receiver at any point in time. This high-frequency signal is converted to the intermediate frequency range (IF) in the receiver, and this IF signal is passed to the diversity processor for the recognition of interference. If interference is recognized, switching signals for switching to a different antenna signal are derived in the diversity processor. In this way, audio-frequency interference caused by multi-path reception can be avoided, if adequate signal reception conditions are present.
The scanning antenna diversity systems of the prior art, such as that known from German Patent DE 44 03 612, receive interference at an antenna due to the superimposition of several partial waves having different amplitudes, phase differences and time differences at the reception location. The resulting level collapses are correlated with frequency interference dispersion peaks, and cause undesirable linear signal distortions as a function of the modulation content in the audio frequency range. If a certain predetermined frequency interference dispersion peak is exceeded, i.e. if an interference-related amplitude modulation is exceeded, the system's interference detector recognizes interference directly, and causes a switch to be made to another available antenna signal, or to a different linear combination formed in an antenna matrix. In this manner, all of the available RF signals are checked for interference and switched through to the receiver, one after the other. The interference energy taken up during the interference recognition time becomes audible, and further reduces the audio signal quality as this search process repeats itself in rapid sequence, and more interference energy falls into the audio channel due to an extended interference recognition time. Therefore the interference recognition time must be made as short as possible. These processors indicate the interference particularly accurately if they are set for simultaneity of the interference in the frequency deviation and the interference amplitude modulation
Prior art diversity processors having a momentary interference indicator, recognize interference if, for example, the frequency deviation threshold or the amplitude interference modulation threshold is exceeded. Noise interference is recognized in the momentary interference indicator only if the actual momentary value exceeds the predetermined threshold by which the interference is being measured. The minimal interference recognition time for a sudden interference that exceeds the predetermined thresholds, e.g. on the basis of adjacent channel, same channel, or intermodulation interference, is limited by the bandwidth of the intermediate frequency channel in processors of this type, and amounts to 30 to 50 μs. An interference recognition time of <100 μs can therefore be assured. For interference intervals of the high-frequency carrier, or intermediate-frequency carrier, at values between 6 dB and 12 dB, the actual value of a frequency deviation of 75 kHz, for example, is connected with undesirable long recognition times.
Particularly in the case of stereo reception, such (S/N)IF values acoustically already result in a clearly noisy signal and are significantly too long to achieve a satisfactory diversity function for this operating state. In the case a prior art interference detector, it is therefore practical if the frequency deviation threshold is regulated as a function of the actual average frequency deviation, in accordance with frequency modulation. Even in the case of slight average frequency deviations (e.g. program contents having a low volume) and a frequency deviation threshold regulated to 40 kHz, for example, the response time is typically 500 μs at a signal of (S/N)IF=9 dB and typically 10 ms at (S/N)IF=12 dB. There are therefore reception situations in which the actual reception signal is clearly noisy, and the interference detector is too slow in time in order to recognize interference, and switch the high-frequency reception signal to a better reception signal. These response times all exceed the tolerable measure for guaranteeing perfect reception behavior in the presence of a noisy signal. It is true that the bandwidth of the IF channel is large enough so that the aforementioned interference caused by adjacent channel, same channel, or intermodulation interference can be recognized at a sufficiently early point in time, but because of the particular statistical properties of a noisy signal, an interference detector of this type is suitable for the recognition of noise only under certain conditions.